1. Field of the Invention
This invention relates to digital phase comparators and to phase comparison radio navigation systems using digital phase comparators.
2. Prior Art
In some types of phase comparison radio navigation systems, the signals to be compared in phase occur simultaneously and it is thus possible to use one waveform as a reference for measuring the phase of the other waveform. The present invention is concerned more particularly however with the measurement of the phase relationship between signals which do not occur simultaneously. Such a requirement arises for example in phase comparison radio navigation systems, such as the Decca Hi-Fix and Sea-Fix (Decca is a Registered Trade Mark), in which signals of the same frequency are radiated sequentially from spaced stations and their phase relationship determined at a receiver. In each of these systems, a signal is radiated firstly from a master station, then from the first slave station and then from a second slave station. These signals are radiated in a predetermined phase relationship and are of the same frequency. At a receiver, the phase relationship between these signals is determined in order to provide information determining the position of the receiver. If the master and the two slave stations are spaced apart at fixed locations and the receiver is on a vehicle or other movable body then such a system forms a hyperbolic navigation system. However, in the particular systems described above, the receiver and the master station may be carried on one vessel which makes use of two slave transmitters at fixed spaced locations. Such a system is known as a two-range system since each of the phase relationships between the signals from the master and a slave station is a measure of the range between the vessel and that slave station.
In the Decca Navigator system, there are sequential transmissions for lane or zone identification purposes, which transmissions can be used, independently of the other transmissions, for position fixing and the discriminator of the present invention may also be used for determination of phase relationship between such sequential transmissions.
In systems in which the signals are radiated in sequence, the practice heretofore has been to have, in the receiver, an oscillator which is phase-locked to the received signals from the master station. Signals from the slave station, when they are received, are compared in phase with the simultaneously-available signals from the oscillator to give the required phase measurements.
It is convenient to indicate phase relationships in decimal digits on digital displays; this can readily be done in the case of simultaneously available signals by determining the time interval (using a clock pulse generator and counter feeding a digital display) between a point on a cycle of one signal and a corresponding point on a cycle of the other signal. It is convenient to use the zero-crossing points as these are accurately identifiable. The present invention however is concerned with a phase discriminator which can determine the phase relationship between signals which are not simultaneously available.